1. Field of the Invention
Methods and apparatuses consistent with the present invention relate to transmitting and receiving multicast packets over a wireless communication network, and more particularly, to a media access control (MAC) structure which simultaneously transmits a retransmission request for a multicast packet via a multipath fading channel.
2. Description of the Related Art
As satellite broadcasting and digital multimedia broadcasting (DMB) services are provided, and interest in broadcasting services in shadow areas increase, the melding of broadcasting, where a broadcasting service and a communication service are fused, is under brisk discussion. Particularly, as one discussion of broadcast fusion, a study for providing a broadcasting service using a related art communication network, i.e., a wireless communication network, is being considered.
The broadcasting service transmitting identical data to unspecified individuals is closely associated with a multicast environment. However, the identical data is transmitted to a plurality of nodes in the multicast environment, and therefore a proper feedback from each receiving node may not be received. Accordingly, an automatic repeat request technique may not be utilized in the multicast environment. The automatic repeat request technique is widely used for assuring reliability of transmission data in a data communication system which transmits the data per frame or per packet.
Due to the above described reason, a method of increasing reliability of transmission data has been used in a related art MAC by transmitting multicast data at a low transmission rate using a high transmission power, without a feedback from a receiver. However, a great amount of transmission resources may be unnecessarily wasted to transmit the data in the related art method. In addition to this, it takes a longer time to transmit the same amount of data, therefore consumption of transmission power is great. Also, the related art method is not suitable for high speed data transmission.
In order to solve the problems of the related art MAC, new techniques capable of minimizing the waste of communication effectiveness have been provided when there is no feedback from the receiver. One of the new techniques is Receiver-driven Layered Multicast (RLM). According to the RLM, in a multimedia application, i.e., when multi-layered data having different levels of importance are simultaneously transmitted, one level of important data, such as control information, is transmitted via a transmission channel whose reliability is comparatively higher, and data having comparatively lower importance, such as multimedia data, is transmitted via a transmission channel whose reliability is comparatively lower.
The RLM indicates a technique where nodes whose transmission paths are in comparatively better condition simultaneously receive data, which is transmitted with lower reliability, while allowing as many other nodes as possible to receive data with higher reliability. However, the quality of communication in the RLM system decreases since nodes with poor receiving capabilities frequently cannot receive data, and therefore the data is transmitted with lower reliability.
Other methods, such as Pseudo-Automatic Repeat Request (ARQ) and Layered ARQ, have been provided to solve the problems associated with the related art MAC.
In the Pseudo-ARQ and Layered ARQ techniques, data is transmitted in low reliability via a basic channel by a sender, first recovery data is transmitted via another channel after a delay of a predetermined time, and second recovery data is transmitted via another channel after a subsequent delay of a predetermined time. Namely, recovery data is simultaneously transmitted via a plurality of channels without a retransmission request. In this instance, in a receiver, initial receiving data may be recovered using the first or the second recovery data received in the time difference when an error is detected in data received via the basic channel.
The above described method may reduce power consumption of the receiver in comparison to the related art MAC by constantly transmitting data while having high reliability. However, since the first and the second recovery data are required to be additionally transmitted by the sender, there is no advantage with regard to the power consumption and transmission capacity, in comparison to the related art MAC.
In order to minimize waste of communication effectiveness, new methods capable of properly receiving a feedback from a receiver in the multicast environment have been provided, in addition to the above described method. When the feedback is transmitted from the receiver in the multicast environment, various problems may occur. For instance, a collisions may occur among retransmission requests when the retransmission requests are simultaneously transmitted from a plurality of receivers, which have detected a transmission error included in a multicast packet, and the overhead related to the network operations for preventing the collisions may increase as the number of receivers in the network increase.
In a wired communication environment, each of the receivers transmits the retransmission request after a random backoff period when the retransmission requests are simultaneously transmitted from the plurality of receivers, and the receiver's own retransmission request is suspended after detecting another receiver's transmitting of the retransmission request at the same time. However, the above described methods may not check transmission contents of other nodes, and may not be applicable to a wireless communication network incapable of checking a collision of a packet.